In recent years, pheromones have become very important in the research of methods useful to control the insects causing infestations of agrarian cultivations. As compared with conventional agents, they offer the advantage of an outstanding selectivity for one species only or for a restricted number of closely related species, without affecting other non-infesting species. It is, therefore, at least theoretically possible to fight a certain pest with the aid of pheromones with minimal disturbance of the ecological equilibrium.
Pheromones are secreted outside the insect body and, depending on the type of reaction they cause, can be divided into aggregation, tracing, sexual, alarming, etc., pheromones.
The most diffused and interesting pheromones, due to the possibilities of application in the control of insects, are the sex pheromones which are most frequently secreted by females, but also by males, and attract the individuals of the opposite sex for copulation. The use of pheromones for controlling the insects is based on the principle that little amounts of such compounds, obtained by synthesis, cause the same reactions as are induced by the male or female insects secreting the natural attractant.
In practice, the synthesized sex pheromones are used both to survey the development of the harmful species population and to control the harmful species by mating disruption. The former type of application (monitoring) permits to follow, by means of periodic samplings with small cage-traps, the density variations of the harmful insect population in order to forecast the time in which the "harmfulness threshold" will be reached. In the latter type of application, the sex pheromones are used to partially or fully substitute insecticides and to directly control the insects by modifying their behavior (mating disruption).
The techniques utilized to this last-mentioned purpose are two: mass trapping and confusion. The former technique (mass trapping) is directed to attracting and catching as many insects as possible by means of small cage-traps. The latter technique (confusion technique) consists in spreading the pheromone in the atmosphere in such way as to render the males or females incapable of "feeling" and locating the individuals of the opposite sex, so hindering copulation.
In practice, the attractant can be diffused by distributing the product in various properly spaced points of the concerned area, or by uniformly spraying it on the whole cultivation. In the first case use is made of evaporators containing the pheromone, which is included or incorporated in materials of various nature suited to cause volatilization to occur at a proper and constant rate. Such methods, however, are rather expensive because of the high cost of both evaporators and labor.
A less expensive and complicated method is that of distributing the pheromone all over the area by atomizing it from the ground or in the air and using special controlled-release formulations.
Some known systems of slow-release formulations are aqueous suspensions of pheromone-containing microcapsules having walls made of polyamides (U.S. Pat. No. 3,577,515) or of gelatin (U.S. Pat. Nos. 2,800,457; 2,800,458), or they may be multilayer polymeric systems incorporating the pheromone, (A.C.S. 33, 1976, pg. 283) or hollow fiber systems consisting of a capillary from an open end of which the pheromone is released and caused to volatilize (U.S. Pat. No. 4,017,030).
Such systems are particularly complicated as regards both the preparation and, chiefly, the subsequent distribution in the field.
A further drawback of those known systems is that of providing a non-constant pheromone release kinetics, just due to the model of the capsule.
The rate at which the pheromone is released is affected not only by the amount of the active compound and the chemical composition of the capsules and of the other formulation components, but also by environmental factors, such as temperature, light, humidity.
A desirable characteristic of a formulation emitting the pheromone in an amount sufficient to permeate the atmosphere and to obtain the effect of hindering copulations is a controlled, complete and constant release over a proper stretch of time.